Cack meaning shit, faeces or poo. Gash being a womans reproductive organ or vagina. Cack gash invariably means a vagina that has been covered in a man or womans shit. This usually happens when a man, after deciding against sex, opts to shit over the ladys vagina. The lady can become incensed and throw fits of rage as a result.

I gave Katie a cack gash last night, she cried for sixteen hours

girl : i want to make sweet love baby
boy : i dont, i want to shit all over your gash bitch!